This project focuses upon analysis of synaptogenesis in (anatomically) the best documented piece of visual neuropile, that of the lamina of the fly's optic lobe. The purpose is to understand the determinants of synapse formation as the strategy by which to unravel retinal circuitry. Serial thin section electronmicroscopy, supplemented by other more specific EM techniques, will be used to sample populations of synapses as they undergo transformation to their mature counterparts during the terminal stage of a chain of events in neurogenesis already extensively characterized in this developmentally ideal animal. The study concentrates upon the photoreceptor tetrad synapses, for reasons of their numerical prepondence and multiple contacts. Similarities between these synapses and those of vertebrate photoreceptors, notably in the sequential construction of both types of synapse during development, emphasize the more than formal comparison between the former's neuropile and that of the vertebrate outer plexiform layer. The synaptogenesis project is the logical antecedent of a potentially much wider proposed study upon the examination of the lamina of the closely-related Drosophila, whose visual mutants provide a continuing source of genetically imposed perturbations in neural architecture with which to test theories of synapse formation. Examination of synaptogenesis in these mutants allows inference of their point of departure from the normal developmental programme. Derangements in the product of synaptogenesis, i.e. adult connectivity, allow on the other hand physiological inferences (by comparison with the functional impairment by which the mutant was first selected), inferences which will be strengthened by correlation between the time course of normal synaptogenesis and onset of function as revealed by the developmental emergence and waveform of the ERG.